After sugar cane has been cut and pressed at the mill to remove sugar, the crushed plant stalk material that remains is called bagasse. Typically, the raw bagasse consists of about 48% fiber, 50% moisture and 2% of soluble matter such as residual sugar and other organic and inorganic substances. Once regarded as a waste material, bagasse is now being looked to as a source of energy to operate plant boilers and the like. However, in its raw form the bagasse is a low-grade fuel that requires large furnaces because of the large volume of flue gases produced, and typically it will have a heat content of 2,005-2,340 kilocalories per kilogram. While useful, the raw bagasse is thus not an especially desirable fuel.
It has been found, however, that if the moisture content of the raw bagasse is lowered, its desirability as a fuel can be greatly increased. Thus, efforts have been made to dry the raw bagasse, and commonly the hot exhaust or flue gases from the sugar factory boilers or similar sources are utilized for this purpose. In the usual arrangement, the raw bagasse is placed in a large dryer to which the hot exhaust gases are admitted, with the raw bagasse being agitated as the gases pass therethrough. This process can lower the moisture content of the raw bagasse by several percentage points, making it a more desirable fuel.
With the high cost of conventional energy, attention has recently been directed toward producing a more efficient fuel from bagasse. It has been found that if the moisture content of the raw bagasse is lowered sufficiently and if the particle size of the material is proper, the bagasse can then be pelletized to form a fuel with relatively high BTU value, compared to the raw bagasse. To achieve this, the raw bagasse first undergoes a primary or initial drying cycle done much as in the past, to produce primary dried bagasse. A portion of the primary dried bagasse is thereafter passed through a secondary drying cycle, to further lower its moisture content and produce secondary dried bagasse suitable for pelletizing.
In processing the bagasse it is still desired to utilize the hot exhaust gases to produce both primary and secondary dried bagasse for overall efficiency in the energy cycle. However, difficulties have been encountered in doing this in an efficient manner.
Part of the problem flows from the fact that the hot exhaust gases actually are relatively low in temperature, usually falling into the range of between about 375.degree. F. and 425.degree. F. This has required drying drums that are large in dimension, to lower the moisture content of the raw bagasse. More particularly, in the past two separate drying operations with separate equipment have been required to produce both primary dried bagasse and secondary dried bagasse, with the processing being done in a sequential manner. The equipment required occupies considerable space, is expensive, and most importantly does not utilize the hot exhaust gases in the most efficient manner.
The need thus exists for a new method and apparatus for producing primary and secondary dried bagasse, the latter being suitable for pelletizing, designed to make maximum efficient use of the energy provided by the relatively low temperature exhaust gases of a sugar factory or the like so as to maximize the overall energy cycle efficiency and minimize the need to utilize oil and other conventional fuels. The present invention is intended to satisfy that need.